There is evidence that afebrile women in premature labor can be infected. Organisms are present in the amniotic fluid of 10% of the women in premature labor. Organisms can be isolated from between the chorion and amnion of 60% of the women who deliver prematurely, in contrast to only 20% of women who deliver at term. The major objective of this study is to define the microbiology and histopathology of women in premature labor and to treat the women in premature labor with antibiotics. The specific aims are: 1) define the vaginal/cervical, amniotic fluid and placental microbiology of women in premature labor, 2) study selected microbiologic factors associated with infection, 3) demonstrate that antibiotics given to women in premature labor will reduce premature deliveries compared to placebo treated women, 4) demonstrate that placental and decidual inflammation is highly correlated with both infection and preterm delivery, and 5) evaluate defense mechanisms which may protect against infection. We plan to conduct a case-control study in which cases consisting of afebrile women in premature labor with intact membranes will undergo vaginal/cervical and trans-abdominal amniotic fluid cultures. These women will be placed in a randomized double- blinded trial of intravenous mezlocillin and oral erythromycin or saline placebo. The control group will be stratified into women who deliver at 34-37 and greater than 37 weeks of pregnancy. After delivery, cultures of the chorion-amnion and histology of the placental and endometrium will be performed. To evaluate the effect of infection on pregnancy the presence of vaginal/cervical, endometrial, placental membrane and amniotic organisms and/or inflammation will be compared among 450 causes and 450 control women. We will evaluate whether high organism concentration or organism virulence will positively correlate with infection and whether selected organisms, amniotic fluid bacterial inhibitory factors and serum antibody will negatively correlate with infection. It is hoped that the result of these studies will demonstrate the microbiology and pathophysiology of infection among women in premature labor and that antibiotic therapy will reduce the rate of premature births.